ML19093B082

Response to a Letter of 09/14/1977 Regarding the Potential for a Boron Dilution Accident at Surry Unit Nos. 1 & 2 Due to Injection of Contents of Naoh Tank Into Reactor Coolant System

ML19093B082
Person / Time
Site:	Surry
Issue date:	12/01/1977
From:	Stallings C Virginia Electric & Power Co (VEPCO)
To:	Case E, Reid R Office of Nuclear Reactor Regulation
References
Download: ML19093B082 (7)

Text

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e VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND,VIRGINIA 23261 December 1, 197].

Mr. Edson G. Case Serial No. 424/091477 Acting Director of Nuclear Reactor Regulation PO&M/DLB:dgt United States Nuclear Regulatory Commission Docket Nos. 50-280 Washington, .D. C. 20555 50-281 Attention: Mr. Robert W. Reid, Chief License Nos. DPR-32 Operating Reactors Branch 4 DPR-37

Dear Sir:

This is in response to your letter of September 14, 1977, regarding the potential for a boron dilution accident at Surry Unit Nos. 1 and 2 due to the injection of the contents of the NaOH tank into the reactor coolant system.

In the case cited in your letter, it was concluded that for certain conserva-tive assumptions, the injection of the NaOH contents into the reactor coolant due to the malposition of a single isolation valve could result in reactor criticality with the control rods inserted.

The attached analysis has been completed to determine the potential for this accident at our Surry Unit Nos. 1 and.2. Analysis revealed that multiple

• component failures and/or operation errors combined with unrealistic initial conditions would be required to initiate this accident. In addition, the con-sequences of this accident were found to be acceptable. Based on this analysis, it is concluded that the postulated accident is neither serious nor credible.

No further analysis and no corrective action is considered necessary.

I

~ r ' tr~l yours, I

( )'/2 / J '

/) u/1 yftfW) Ul '/:_~<-,;/

• M. Stallings 1

J Vice resident-Power pp)t, an~ Production Operations Attachment

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ANALYSIS OF POTENTIAL FOR BORON DILUTION OF THE REACTOR COOLANT SYSTEM BY DRAINAGE OF THE CHEMICAL ADDITION TANK Introduction The purpose of this analysis is to investigate the potential for the boron dilution of the reactor coolant by drainage of the NaOH tank into the reactor coolant system. This is in response to an NRC letter dated September 14, 1977 which cited a dilution incident at another operating PWR facility, and requested this analysis for Surry Power Station, unit ~os. 1 and 2.

System Description

Reactor Coolant System, Residual Heat Removal System, and Containment Spray System components and piping relevant to this analysis are shown in Figure 1.

The Refueling Water Storage Tank (RWST) normally contains not less 350,000 gallons of borated water at not less than 2000 ppm. The RWST provides borated water to the safety injection system and the containment spray system under accident conditions. In addition, the RWST provides borated water to fill the reactor cavity during refueling operations. The RWST water temperature is main-tained at approximately 40 degrees F by circulation through a cooling loop which contains two recirculation pumps, two chilled water cool~rs and two mechanical refrigeration units. The chemical addition tank (CAT) contains n_ot less than 3360 gallons of solution with a NaOH concentration of not less than 18 percent by weight. The CAT provides a balanced gravity feed of NaOH solution for iodine removal during containment spray operation.

The Residual Heat Removal (RHR) system provides cooldown below approximately 350 degrees F and continuing removal of decay heat during unit shutdown. This system includes two pumps, two heat exchangers, and motor-operated valves for isolation from the reactor coolant system. A reactor cavity dewatering line (RH-20) runs from the discharge of the RHR pumps to the discharge of the refuel-ing water recirculation pumps. ~his line, RH-20, is used to return water from the reactor cavity to the RWST following completion of refueling.

System Operations Under normal operation the valve lineup is as shown in Figure 1.

Suction to the refueling water circulating pumps is normally from the upper tap through valve 1-CS-26. The lower tap through valve 1-CS-27 is only opened when higher flow rates through the cooling loop are necessary for mixing prior to chemistry sampling or for accelerated cooldown as might be required prior to unit startup.

The chemical addition tank outlet valves, MOV-CS-102A and Hov~cs-102B, are normally closed. They are opened only on containment spray initiation and once per month for valve testing. Testing of .each involves closing of the upstream and downs trea,m isolation valves, opening and closing the MOV; and reopening the isolation valves. The valves are tested one at a time to ensure that a path for

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NaOH addition is always available.

Under normal operations, the RHR system is shutdown and isolated from the reactor coolant system with valves MOV-RHR-1700, 1701, 1720A and 1720B closed.

During shutdown, RHR is in service with these valves open, 1 pump and 1 or 2 heat exchangers in service. Valve HOV-RHR-100 is shut during normal operation and during normal shutdown. This valve is opened only for a short period during refueling when the reactor cavity water is returned to the RWST.

Reactor Core Conditons - Shutdown Margins During refueling shutdown, a shutdown margin of at least 10;~ reactivity is required. During cold shutdown a shutdown margin of at least 1% reactivity is required. Administrative limits require a 2% shutdown margin for cold shut-down. Under the worst conditions, i.e. beginning of core life, shutdown banks out, using our administrative limit of 2%, a boron concentration of not greater than 1400 ppm is required. This value of 1400 ppm represents the maximum boron concentration which would be required to maintain a 2% shutdown condition at any time in core life, with the shutdown banks out.

Dilution Accident Considerations The following combination of event*s would be necessary to".'1ead to a dilution of the reactor coolant system by drainage of the chemical addition tank.

1) Malposition of either of the chemical addition tank discharge valves, MOV-CS-102A or 1021

2) A difference in level between the CAT and the RWST, i.e. RWST must be below normal level

3) Valve l-CS-27 must be open

4) The RWST recirculation loop must be in normal lineup

5) Valve MOV-RHR-100 must be open

6) Both RHR pumps must be shutdown during the above events, after which one or both pumps must be restarted to carry the dilution into the reactor coolant system This series of events would allow flow from the CAT to the RWST, dilution of the RWST, flow from the RWST through valve l-CS-27 into the recirculation loop, to the RHR *system through MOV-RHR-100 and then into the reactor coolant system.

Analysis of Events Following is an analysis of the potential for each of the events listed above.

Opening of CAT Discharge Valves Malpositioning of either valve HOV-CS-102A or MOV-CS-102B is necessary to

l e e establish a flowpath from the CAT to the RWST. In the case cited in the NRG letter, the corresponding valve was malpositioned during a valve cycling test.

Our procedure for this test requires the closing of both the upstream and down-stream isolation valves prior to cycling the NaOH tank discharge valve. Thus,

• a flow path to the RWST would not be established without a violation of the test procedure. Accidental opening of these valves is considered extremely unlikely due to their location on the safeguards area of the control board.

Flow to the RWST If a flowpath to the RWST is established, the rate of flow and the amount discharged to the RWST would depend on the relative levels of the RWST and the chemical addition tank. Under normal conditions, the RWST would be 100% full.

If either NaOH tank discharge valve opened under this condition, there would be only a slight flow of NaOH into the RWST, this being due to the higher speci-fic gravity of the NaOH solution. Ultimately, due to specific gravity difference, the NaOH would settle to the bottom of the RWST. Assuming complete stratification occurred, the NaOH would settle to the bottom 5 inches of the RWST, 15 inches be-low the lowest outlet to the recirculation loop. Thus, the n~lpositioning of either NaOH tank discharge valve with a 100% level in the RWST does not present a dilution problem.

The assumption of an RWST level other than 100% presents -certain conflicts with other assumptions specified in the NRG analysis request. The only operation during which RWST level is reduced below 100% is for .filling the reactor cavity during refueling. This is inconsistent with the assumpd,on that the reactor vessel is drained to the nozzles as specified in the NRG request. There is no normal operating condition where the RWST level is below 100% and the reactor vessel is drained to the nozzles. However, while this is not realistic, it is conservative, and simplifies analysis.

The worst initial level in the RWST for this accident is in the range of 15" to 20 11

• The outlet to l-CS-27 is located at the 20 11 level. RWST levels above 20" are not specifically addressed because to have a level above 20", l-CS-27 must have been closed.

• The only other outlet to the recirculation loop .is then through l-CS-26 which is almost at the lb0% level. As discussed previously, opening of the NaOH tank discharge valve with a very high level in the RWST does not present a dilution problem.

Figure 2 summarizes calculations of the release from the RWST resulting from drainage of the CAT for RWST levels from 15i; to 20". At 20 11 of level 3670 gallons with a boron concentration of 1588 ppm are released. For levels below 20 11 , the dilution of the RWST water is greater. At 15" or below, no re-lease occurs because the water added is not sufficient to reach the outlet at 20 11

• Note that this calculation assumes complete mixing which is conservative.

Settling of the heavier NaOH solution would reduce the amount which overflows through l-CS-27. Since the water in the RWST begins overflowing to the recircu-lation loop as soon as drainage begins and much of the NaOH solution is still in the CAT as overflow continues, the actual boron concentration leaving the RWST will be significantly higher than calculated.

Flow from RWST to RHR System In order for the borated water to leave the RWST and flow into the reactor

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e coolant system via the RHR system it is necessary to have the RHR system lined up for operation, with both RHR pumps shutdown and MOV-RHR-100 open.

In the shutdown condition, one RHR pump is run at all times. If HOV-RHR-100 is opened with one or both RHR pumps running, water would flow to the RWST from the. RHR system.

Valve MOV-RHR-100 is only opened during refueling for dewatering the refuel-ing cavity, at which time one RHR pump would be running. Failure of a running RHR pump would quickly cause an RHR low flow alarm, at which time the operator would start the other pump.

It is therefore unlikely that MOV-RHR-100 would ever be open at the same time that the RHR system was aligned for operation with both RHR pumps shutdown.

Reactivity Effects As explained above, the quantity and boron concentration of water leaving the RWST varies depending on RWST level. Figure 2 summarizes this data. Also in Figure 2 is a tabulation of the ultimate effect of this water reaching the primary. This calculation assumes an initial concentration of 1400 ppm in the primary. As discussed previously, 1400 ppm is the maximum boron concentration which would be required to maintain a 2% shutdown condition at,,..*any time in core life. Note that in every case a negative reactivity addition is the result. For*

lesser initial reactor coolant boron concentrations, a greater negative reactivity addition would result. For boron concentrations above approximately 1480, this accident would result in a minor net dilution (for example, at 2000 ppm, a dilution of 50 ppm would occur). However, this could never reduce the boron concentration below 1480 ppm which is well above the concentration required for 21o shutdown margin. Therefore, the consequences of this accident are not serious.

Conclusion The sequence of events necessary to cause a reactor coolant boron dilution by drainage of the sodium hydroxide tank were listed on page 2. Subsequently, the potential for each event was considered. In addition, the consequences of the accident were analyzed and determined to be acceptable.*

The NRC letter addressed a situation where a single malpositioned valve could result in a dilution. This is clearly not the case at Surry Unit Nos. 1 and 2.

At Surry this accident would require the malpositioning of at least 2 valves, the failure of at least one pump, combined with a series of plant conditions which, while possible, do not occur simultaneously under any known operational mode or evolution.

It is therefore concluded that this accident is neither serious nor credible.

No further analysis and no corrective action is considered necessary.